It is known in the Art to provide uninterruptable power supply (UPS) systems, which comprise multiple parallel converters. The converters are responsible for converting source power into output power as required for a load. Hence, the converters are typically connected to a primary power source for normal operation and a secondary power source, which powers the load in case of a failure of the primary power source. Primary and secondary source can be any kind of source, which can be provided individually for each converter, or which can be provided commonly for groups of converters or even all converters. Typically, the primary power source is an AC source, and the secondary power source is a DC source.
The typical design of the converter comprises a DC link, which is connected to a first and second input converter unit connected to the primary and secondary power source, respectively, and an output converter unit, which provides the output power as required for the load. Hence, the output converter unit can be a DC/AC-converter or a DC/DC-converter, depending on the type of load.
The implementation of the UPS system with multiple parallel converters has many advantages. Amongst others, some of the advantages of a system design with multiple parallel converters are scalability of the UPS system for varying loads and redundancy of converters in case of failure. Furthermore, depending on the actual load, converters which are not required can be operated in standby or even shut down to reduce energy consumption of the UPS system.
Nevertheless, such UPS systems have system efficiency curves, i.e. system efficiency vs. load level, that depend heavily on load level. At light load for individual UPS the unavoidable base load of having the UPS converters and auxiliary functions just operating can be a reasonable fraction of total losses. In addition, also main circuit losses depend on load, for instance simply on a current squared basis for a resistive load or on a filter current envelope in case of discontinuous to continuous currents.
Installations of UPS systems strive for minimal overall losses in order to reduce operational costs. These costs are not only based on simple cost of electricity of the UPS system, but also includes operational costs for e.g. cooling heat from the losses of the UPS system. Furthermore, depending on the losses, also the entire design of the UPS system can have increased costs, e.g. when the system installation of providing for instance for an increased level of cooling is to be modified.
Energy consumption of the UPS system is nowadays reduced by operating the converter in different operational states. Hence, a system level control of the individual converters of the parallel system enables to operate them either supporting the load or in a ‘ready’ state, where the converter is actually not supporting a load. Hence, converters in ‘ready’ state reduce the overall energy consumption of the UPS system. Depending on the implementation of different ready states, the converters can be turned on to support the load in a short time as required.
The criterion for operating individual converters in the different operational states is system load level, which defines the number of individual converters required to support the load. Furthermore, a level of redundancy can be defined. The required number of converters support the load in active state, and the rest of the converters are dormant. The UPS system continuously adapts the number of active converters to any change in load level by adding/removing active UPS.